Photographic silver halide materials are often chemically sensitized with one or more compounds containing labile atoms of gold, sulfur or selenium and the like to provide increased sensitivity to light and other sensitometric properties. Examples of typical chemically sensitized photographic silver halide emulsions are described in, for example, Research Disclosure, Item No. 308119, December 1989, Section III, and the references listed therein (Research Disclosure is published by Kenneth Mason Publications Ltd., Dudley Annex, 12a North Street, Emsworth, Hampshire PO 10 7DQ, England.)
Gold compounds may contain gold in the (I) or (III) oxidation state. However, those in the (I) oxidation state are preferred because those gold compounds in the (III) oxidation state may undergo side reactions that, for example, oxidize gelatin or other components in photographic emulsions. Among gold(I) compounds, trisodium aurous dithiosulfate is commonly known as a chemical sensitizer, but is not universally applicable because of the disadvantages this compound provides. In particular, this gold(I) compound contains two thiosulfate ions that are bonded to gold. These ions may also undergo sensitization reactions in addition to the gold in a photographic silver halide emulsion. Therefore, this gold(I) compound is not appropriate in silver halide compositions in which an amount of sulfur less than a 2:1 molar ratio with gold is desired for chemical sensitization, and not appropriate in silver halide compositions in which sulfur or selenium sensitizers other than thiosulfate are desired, such as a silver halide composition containing thioureas as described in U.S. Pat. No. 4,810,626 of Burgmaier et al.
Gold(I) compounds are known that do not contain thiosulfate ligands or other ligands possessing labile sulfur. However, many such gold(I) compounds are not useful as chemical sensitizers for photographic silver halide materials because their dissociation constants are too high and provide low stability. Such gold(I) compounds are susceptible to disproportionation or reduction by gelatin components, especially those in photographic silver halide emulsions. Many gold(I) compounds are not sufficiently soluble to be easily dispersed in a photographic silver halide composition in a uniform and controllable manner.
One gold(I) compound that has been proposed is a gold(I) thiolate as described in U.S. Pat. No. 3,503,749 of Tavernier et al. This compound contains a sulfonic acid sodium salt substituent on the thiolate ligand to impart water solubility. However, the process for preparing such gold(I) compounds involves use of gold fulminate that is explosive and thus not desirable for practical use.
Other gold(I) compounds, such as those containing alkyl or aryl thiolate ligands are also not useful because the alkyl or aryl thiolate may be readily displaced from the gold compound by protons, as described in, for example, G. E. Coates, B. Kowala and J. M. Swan; Aust. J. Chem., 19, 539 (1966).
Thus, there has been continuing interest in synthesizing new gold(I) compounds which have good stability and solubility properties and which contain ligands that will not undergo further reaction after dissociating from the gold(I) ion or interfere with the emulsion in any adverse way.
A well-known phenomenon in coordination chemistry that enhances the stability of metal ion complexes is the chelate effect (J. E. Huheey, "Inorganic Chemistry" 2nd ed., Harper & Row, New York, 1978, pp. 481-482). A chelate is a ligand that provides two or more coordination sites for a metal ion. A bidentate ligand, for example, will provide a lower dissociation constant (greater stability) for the complex ion than two separate monodentate ligands of the same type. Should one donor group of the bidentate temporarily dissociate, the second donor group will hold the ligand molecule in place so that it does not drift away, thereby favoring recombination.
Most bidentate ligands coordinate to metal ions in a cis configuration. But gold(I) normally requires linear, twofold coordination. Few bidentate ligands can extend for the trans coordination required by gold(I). Consequently, bidentate ligands generally form polymeric chains with gold(I) and mononuclear gold(I) chelates are quite rare. Even if there is a long separation between the two donor groups of a bidentate ligand that could accommodate the 180.degree. C. coordination of gold(I), it is most probable that highly fluxional ligands will still form polymeric chains. These polymeric materials are not desirable because they tend to be insoluble and they also can have variable stoichiometry due to variable chain lengths. To increase the probability of forming a mononuclear chelate, the ligand molecule must be rigid, or at least have limited degrees of freedom, so that the two coordination sites are pointing at each other in close proximity. This situation could be provided by macrocyclic ligands and, in particular, macrocyclic thioether or selenoether compounds containing at least two sulfur and/or selenium atoms in the ring.